By default, open-subscription lists at UGA are set up to send probe messages to subscribers every six months to verify that their addresses are still valid.

The subscriber does not have to take any action in response to the probe to stay on the list, but if repeated probes fail, he will be automatically deleted from the list.

See the List Owner's Manual http://www.lsoft.com/manuals/1.8d/owner/owner.html#4.9.1 for more information on probes.